Biosensors that utilize a field effect transistor have heretofore been proposed (see Patent Literatures 1 to 3). Generally, in such a field-effect transistor biosensor, source/drain electrodes and a channel are formed on an insulating film formed on a semiconductor substrate, and in many cases, a reaction field is disposed on the channel or insulating film on the semiconductor substrate. Target recognition molecules are often immobilized on the reaction field.
The target recognition molecules immobilized on the reaction field are allowed to recognize a target substance. A source-drain current upon target recognition is measured to determine the presence or concentration of the target substance supplied to the reaction field.